This invention relates to inclinometers, and is particularly concerned with inclinometers which are suitable for use in downhole instrumentation of oil and gas wells during the process of drilling and during directional survey work in completed wells.
Directional drilling of oil and gas wells is a widely employed technique; it is routinely used to achieve drilling coverage of a maximum area from a single offshore drilling platform. In the course of directional drilling it is necessary to measure and control the angle at which the bit is addressing the strata substantially continuously. In addition, wells which are intended to be substantially vertically drilled sometimes deviate from the vertical for various reasons and it is necessary to determine the actual course of the well after completion. Inclinometers are used to make such measurements.
The environment at the bottom of a well being drilled is a severe one for an inclinometer. In addition to elevated temperatures (typically in the neighborhood of 300.degree.F.), severe vibrational accelerations are often encountered, for reasons outlined below.
It is becoming increasingly popular to perform high angle directional drilling by the use of downhole mud motors or mud turbines, instead of applying the drilling torque at the surface and transmitting it through the drill string to the bit. With mud motors, the drill pipe is not rotated, and is hence not subjected to continuous and severe bending stresses inherent in rotation of a shaft in a curved hole. In addition, the problem of key seating is eliminated, since the drill pipe is not in rotating contact with the walls of the bore hole.
In mud motor or mud turbine directional drilling, the primary determinant of the angle at which the bit addresses the strata is the degree of bend provided in a section of drill pipe (a "bent sub") at the lower end of the drill string just above the motor. But it is not the sole determinant, because the reactive torque applied by the bit tends to rotate the bent sub away from its planned position. Since the reactive torque is not constant, because of variations in what the bit encounters and variations in mud flow through the motor, the angular displacement of the bit and bent sub is not constant. It therefore becomes necessary to determine, substantially continuously, the relative positions of the high side of the hole and the bent sub so that a corrective torque can be applied to the drill string at the surface to bring the bit to, and hold it at, the desired angle of address.
Inclinometers, together with associated telemetry and control equipment, are used for this purpose. Because of their application, such inclinometers are often called "top hole indicators."
From the foregoing outline, it can be seen that a top hole indicator is necessarily subjected to severe vibrational conditions by the interaction of the bit, hole bottom, and mud motor.
Inclinometers employed heretofore have relied on pendulums rotatably or otherwise mounted on shafts and ancillary equipment such as bearings. In practice, such devices are less than satisfactory because the pendulum must have an appreciable mass in order to maintain its vertical position, and the acceleration forces on this mass caused by the vibratory environment rapidly wear the support system of the pendulum, thus inducing uncompensated errors into the measurements, and rendering the equipment unfit for use in an undesirably short time.